1. Field of the Invention
The present invention relates to a liquid crystal device.
2. Description of the Related Art
As a method for achieving switching between a colored state and a transparent state, a display device of guest-host mode is known. This display device of guest-host mode is able to achieve bright display and is expected as a display device suitable for a reflection type.
As disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 7-104297 and 7-199233, in a display device of guest-host mode, in order to render it in a colored state at the OFF time of voltage (at the initial time) (i.e. initial coloring mode), a liquid crystal with positive dielectric anisotropy (Δ∈) is combined with a horizontal alignment film. At the time when no voltage is applied, a host liquid crystal is horizontally aligned against electrodes due to the horizontal alignment film, and following the alignment of the liquid crystal, a dichroic dye is also horizontally aligned to exhibit a color. When voltage is applied, the host liquid crystal with positive dielectric anisotropy (Δ∈) is vertically aligned against the electrodes, whereby the light transmittance increases.
On the other hand, for the purposes of use in, for example, light modulation of a windowpane, security, interiors, advertisements and information indication panels, there are uses of a liquid crystal for limiting the transmission of light or shielding light. For such an application, it is desirable to perform display of a scattered (light shielded) state at the OFF time of voltage (at the initial time) and achieve switching between a scattered (light shielded) state and a transparent state by electric field driving.
In a driving method of a liquid crystal device, the following methods are disclosed as the method for achieving switching between a scattered (light shielded) state and a transparent state.
For example, there is a method for using a liquid crystal device provided with a liquid crystal layer in which a capsule including a liquid crystal therein is dispersed in a polymer. At the OFF time of voltage (at the initial time), the alignment of the liquid crystal becomes random, and light is diffusely reflected due to a difference in refractive index between the liquid crystal and the polymer to cause scattering (light shielding). At the ON time of voltage (at the time of voltage application), the alignment of the liquid crystal becomes uniform, and the major axis direction of the liquid crystal and the refractive index of the polymer are substantially coincident with each other, thereby producing a transparent state. In this way, the white scattered state and the transparent state are switched by the ON/OFF operation of voltage (see, for example, ICHIMURA, Kunihiro Ed., Development of Chromic Materials, CMC Publishing Co., Ltd. (issued in 2000), pages 226 to 236).
But, in this polymer-containing liquid crystal device, in order to display the colored state, it is necessary to dissolve a dichroic dye in the liquid crystal. However, the dichroic dye is dyed on a capsule film, or the dichroic dye is easily aligned along the polymer film to reduce voltage responsibility, thereby causing a problem that the transmittance at the transparent time is reduced.
As other methods, as disclosed in WO 2002/093241, there is a method for using a liquid crystal device provided with a liquid crystal layer prepared by mixing an uncured ultraviolet ray-curable resin, a polymerization initiator, a liquid crystal and a dichroic dye, and curing the resin upon irradiation with ultraviolet rays. This method is a method in which this liquid crystal device acts on the same principle as in the above-described liquid crystal device because the polymer and the liquid crystal cause phase separation to form an interface between the polymer and the liquid crystal.
However, according to this method, there is involved a problem that the dye is decomposed upon irradiation with ultraviolet rays or due to the polymerization initiator, thereby reducing coloring properties.
In these display methods employing an initial scattering and coloring system using a polymer, in order to increase absorbance scattering and coloring for the purpose of realizing high contrast, when widening of a cell gap may be considered. However, when the cell gap is widened, transmittance at the time of transparent display is reduced and, as a result, the contrast tends not to increase sufficiently. Also, when the cell gap is wide, there is a problem of the driving voltage increasing due to application of the polymer.